Radio-frequency signal frequency conversion device for a low power RF receiver

ABSTRACT

The radio-frequency signal frequency conversion device generates intermediate complex signals (IF) for a correlation stage of a low power RF receiver. In order to do this, the device includes a first selective pass-band filter for filtering radio-frequency signals picked up by an antenna. A frequency synthesizer generates first and second high frequency signals, wherein the frequency of the first signals is higher than the frequency of the second signals. This synthesizer receives reference signals from an oscillator unit. A first mixer unit mixes the radio-frequency signals with the first signals in order to generate frequency-converted signals. A second pass-band filter filters the signals from the first mixer unit, and provides signals to a second mixer unit to mix them with the second high frequency signals. Finally, shaping means for the signals provided by the second mixer unit generate the intermediate signals. The second filter is a not very selective active filter, which is integrated, in an RF/IF integrated circuit with the first and second mixer units, the signal shaping means, and certain parts of the synthesizer and the oscillator unit.

This is a National Phase Application in the United States ofInternational Patent Application No. PCT/EP02/08566, filed Aug. 1, 2002,which claims priority on European Patent Application No. 01203037.5,filed Aug. 10, 2001. The entire disclosures of the above patentapplications are hereby incorporated by reference.

FIELD OF THE INVENTION

The invention concerns a radio-frequency signal frequency conversiondevice for a low power RF receiver, in particular of the GPS type. As afunction of the received radio-frequency signals, the device generatesintermediate signals intended to be processed in a correlation stage ofthe low power RF receiver.

The invention also concerns an RF/IF integrated circuit as part of thefrequency conversion device.

BACKGROUND OF THE INVENTION

The frequency conversion device includes first of all a first pass-bandfilter for filtering radio-frequency signals picked up by an antenna ofthe receiver, said first filter being a passive selective filter foreliminating image frequencies during a first frequency conversion. Thedevice further includes means for generating oscillating signals forgenerating first and second high frequency signals, the frequency of thefirst signals being higher than the frequency of the second signals. Thefirst high frequency signals are mixed with the filtered radio-frequencysignals in a first mixer unit. The signals generated by the first mixerunit have a frequency equal to the result of a subtraction of thefrequency of the first signals from a carrier frequency of theradio-frequency signals. The device further includes a second pass-bandfilter for filtering the signals coming from the first mixer unit and asecond mixer unit for mixing the signals filtered by the second filterwith the second high frequency signals. The signals generated by thesecond mixer unit have a frequency equal to the result of a subtractionof the frequency of the second signals from the frequency of the signalscoming from the first mixer unit. Finally, the device includes means forshaping the signals provided by the second mixer unit for generating theintermediate signals.

In the case of a GPS receiver, the correlation stage has the task ofextracting the GPS messages or data from the intermediate signalsreceived from the device. The messages are transmitted to microprocessormeans of the receiver for calculating the position and time-relateddata. Of course, the receiver has to pick up the radio-frequency signalsof at least four visible satellites for the position calculation.

The frequency conversion device can also be used in any radio-frequencysignal receiver other than a GPS receiver. It may for example be areceiver used in a satellite navigation system of the GLONASS or GALILEOtype. It may also be a receiver used in a mobile telephone network, forexample of the CDMA type (Code-division multiple access).

The use of RF receivers, particularly of the GPS type, is currentlywidespread. This allows a user of such a receiver to be able to take abearing in the direction of a desired target and to find the position ofhis current location. Consequently, it becomes necessary to be able toincorporate an RF receiver in objects which are used daily and which asingle person can easily carry.

GPS receivers can be mounted for example in a wristwatch or a mobiletelephone. However, in order to be mounted in these small sized objects,the receivers have to fulfil certain conditions. On the one hand, thepower consumption of low power receivers has to be greatly reduced,since the objects are powered by batteries or accumulators of smallsize. On the other hand, the number of components of the receiver alsohas to be considerably reduced.

Usually, the frequency conversion devices of RF receivers are designedto perform a triple frequency conversion of the received radio-frequencysignals. An embodiment of such a device according to the prior art isshown schematically in FIG. 1.

With reference to FIG. 1, the frequency conversion device is connectedto an antenna 2 of the RF receiver, particularly of the GPS type, forpicking up RF signals originating from visible satellites. For civilapplications, the carrier frequency of the GPS radio-frequency signalshas a value of 1.57542 GHz.

The radio-frequency signals picked up by the antenna are first of allfiltered and amplified by a first filtering and amplification element101. The radio-frequency signals filtered by element 101 are then mixedin a first mixer 102 with first high frequency signals provided by avoltage-controlled oscillator 109. The signals thus generated by mixer102 are signals whose frequency is equal to the result of a subtractionof the frequency of the first high frequency signals from the carrierfrequency of the filtered radio-frequency signals.

For this first frequency conversion operation by first mixer 102, thepass-band filter of element 101 has to be a selective passive filter ofthe SAW type. Said filter of element 101 has to be sufficientlyselective to eliminate the image frequency of the radio-frequencysignals at the input of the first mixer.

With a frequency of the first high frequency signals equal, for example,to 1.3961 GHz, the frequency of the signals generated by the first mixerhas a value of approximately 179.3 MHZ. Thus the first selective filterhas to be capable of eliminating the image frequency having a value of1.2168 GHz (1.3961 GHz-0.1793 GHz) from the received radio-frequencysignals.

The signals generated by the first mixer 102 are filtered and amplifiedby a second filtering and amplification element 103. The signalsfiltered by element 103 are then mixed in a second mixer 104 with secondhigh frequency signals provided by a first divider 110 connected tooscillator 109. The signals thus generated by second mixer 104 aresignals whose frequency is equal to the result of a subtraction of thefrequency of the second high frequency signals from the frequency of thesignals generated by the first mixer.

For this second frequency conversion operation by second mixer 104, thepass-band filter of element 103 also has to be a selective passivefilter in particular of the SAW type.

The first high frequency signals are divided, for example, by 8 using afirst divider 110 for generating the second high frequency signals whosefrequency has a value of approximately 174.5 MHz. Thus, the frequency ofthe signals generated by the second mixer has a value of approximately4.8 MHz. The second selective filter thus has to be capable ofeliminating the image frequency having a value of approximately 169.7MHz (174.5 MHZ-4.8 MHZ) of the signals generated by first mixer 102.

The signals generated by second mixer 104 are then filtered andamplified by a third filtering and amplification element 105 whichincludes a pass-band filter. The signals filtered by third element 105are mixed in a third mixer 106 with clock signals CLK provided by afrequency divider 115 connected to a reference oscillator 114.

The frequency of the reference signals generated by the referenceoscillator has a value, for example, of 17.452 MHz. This referencefrequency is divided by 4 by divider 115 to generate clock signals atthe frequency of 4,363 MHz. Thus for this third conversion operation,the frequency of the signals generated by the third mixer is close to400 kHz.

The signals generated by third mixer 106 still have to be filtered andamplified by a fourth filtering and amplification element 107, whichincludes a low-pass filter, and be sampled and quantified by a sampleand hold converter 108. This converter 108 is clocked by clock signalsCLK.

In order to generate the first and second high frequency signals, thedevice has a phase lock loop frequency synthesiser 100. This synthesiserincludes voltage controlled oscillator 109, two frequency dividers 110and 111, a frequency and phase detector 112 for comparing the frequencyof the signals originating from oscillator 109, which are divided bydividers 110 and 111, with the frequency of the reference signalsgenerated by reference oscillator 114. The control signals leavingdetector 112 are filtered by a low-pass filter 113 in order to generatea control voltage at oscillator 109 as a function of the comparison ofthe signals provided to said detector 112.

A major drawback of the device of FIG. 1 is that it includes too large anumber of electronic components to achieve the triple frequencyconversion. A large part of the components still has to operate at highfrequency. Consequently, the current consumption of the device is toohigh. One cannot therefore envisage mounting an RF receiver comprisingthe device in an object of reduced size, since said object includes abattery or accumulator of small size. This battery or accumulator wouldbe too quickly discharged during operation of the RF receiver.

Another drawback lies in the use of at least two selective pass-bandfilters of the SAW type, which are expensive and bulky components. Thedimensions of each encapsulated filter is of the order of 5 mm×5 mm×1.3mm, which involves a significant loss of space for assembly in an objectof small size such as a wristwatch or a cellular telephone. It shouldalso be noted that SAW type selective filters generate a gain loss ofthe filtered signals which means that said signals have to be amplifiedfor the subsequent processing operations.

In order to reduce the number of components for the frequency conversionin such a device, it has already been proposed to perform only a doublefrequency conversion instead, of the usual triple frequency conversion.European Patent No. EP 0 523 938 B1, which discloses a radio receiver,can be cited in this regard. Said receiver includes a frequencyconversion device performing a double frequency conversion of theradio-frequency signals received at an antenna.

The frequency conversion device shown in FIG. 1 of this Patent includesa phase lock loop frequency synthesiser 30 formed mainly of a voltagecontrolled oscillator 28, which provides first high frequency signals toa first mixer 14. The first mixer also receives radio-frequency signalsfiltered and amplified by amplification and filtering element 12, whichincludes a selective pass-band filter. The converted signals generatedby the first mixer have a frequency of the order of 200 MHz.

A second amplification and filtering element 16, which includes aselective pass-band filter, filters and amplifies the signals generatedby first mixer 14 in order to provide filtered signals to a second mixer18. This second mixer 18 also receives second high frequency signalswhose frequency is an integer number times less than the frequency ofthe first high frequency signals. The frequency of the signals producedby the second mixer is approximately equal to 26 MHz. These signals andthen filtered and amplified by a third element 20 before being providedto a processor 22.

One drawback of the device presented in this European Patent lies in thefact that it is necessary to use two selective passive filters, whichcan, for example, be of the SAW type. Even if the two mixers and a partof the frequency synthesiser are integrated in the same integratedcircuit, the two selective filters cannot, however, be integrated insaid integrated circuit. Consequently, a significant loss of spaceremains with the use of external filters, and the cost of making adevice with these expensive components remains high.

Another drawback lies in the fact that the current consumption remainshigh, since a large part of the components operates at high frequencyand the selective filters consume an enormous amount. Assembly of such areceiver in an object of small size cannot be easily achieved.

An object of the present invention consists in making a frequencyconversion device that reduces energy consumption, as well as the numberand size of electronic components as much as possible to carry out adouble frequency conversion in order to overcome the drawbacks of thedevices of the prior art. Thus, the RF receiver including said devicecan easily be mounted in an object of small size, such as a wristwatchor a cellular telephone.

SUMMARY OF THE INVENTION

This object, in addition to others, is achieved by the aforecitedconversion device, which is characterised in that the second filter is anot very selective filter.

In accordance with an apparatus embodiment of the present invention, aradio-frequency signal frequency conversion device (3) for generatingintermediate signals (IF) intended to be processed in a correlationstage (6) of a low power RF receiver (1) is provided, wherein the deviceincludes: (a) a first pass-band filter (12) for filteringradio-frequency signals picked up by an antenna (2) of the receiver,said first filter being a passive selective filter for eliminating theimage frequencies during a first frequency conversion; (b) oscillatingsignal generating means (20 to 29) for generating first and second highfrequency signals, the frequency of the first signals being higher thanthe frequency of the second signals; (c) a first mixer unit (14) formixing the filtered radio-frequency signals with the first highfrequency signals in order to generate signals whose frequency is equalto the result of a subtraction of the frequency of the first signalsfrom a carrier frequency of the radio-frequency signals; (d) a secondpass-band filter (15) for filtering the signals originating from thefirst mixer unit; (e) a second mixer unit (16) for mixing the signalsfiltered by the second filter with the second high frequency signals inorder to generate signals whose frequency is equal to the result of asubtraction of the frequency of the second signals from the frequency ofthe signals originating from the first mixer unit; and (f) shaping means(17, 18, 19) for the signals provided by the second mixer unit forgenerating the intermediate signals, wherein the device is characterisedin that the second filter is a not very selective filter.

Furthermore, the above apparatus embodiment of the present invention maybe modified so that the frequency of the first high frequency signals isn times higher than the frequency of the second high frequency signals,n being an integer number selected, in particular, from the range of 50to 100, and is preferably equal to 64.

In a preferred embodiment of the device, the second filter is an activepass-band filter in which the signals generated by the first mixer unitare filtered and amplified.

In another preferred embodiment of the device, the second filter isintegrated in an RF/IF integrated circuit with the first and secondmixer units, the signal shaping means and certain parts of theoscillating signal generating means.

One advantage of this frequency conversion device lies in the fact thata single RF/IF integrated circuit can include the two mixer units, thesecond not very selective pass-band filter, the shaping means for thesignals provided by the second mixer unit to generate the intermediatesignals, and the majority of the oscillating signal generating means.These signal generating means include, in particular, at least onefrequency synthesiser connected to an oscillator unit which provides itwith reference signals. Only a low-pass filter of the synthesiser andthe quartz crystal of the oscillator unit are external components to theRF/IF integrated circuit.

The number and the size of the electronic components of the frequencyconversion device is thus reduced to the minimum. Outside RF/IFintegrated circuit remain only the first passive selective filter of theSAW type, an amplifier for the radio-frequency signals picked up by theantenna of the receiver, the low-pass filter and the quartz crystaldescribed hereinabove.

Another advantage is that the second pass-band filter can be integratedsince it does not need to be selective. This second filter is thuscalled a not very selective filter. The second filter is an activefilter providing amplification of the received signals, which avoidsusing an amplifier to increase the detection sensitivity of theradio-frequency signals. It should be noted that this second filter doesnot need to be selective in the same way as the first pass-band filter,since the frequency of the signals generated by the first mixer iswithin a margin of 50 to 100 times less than the carrier frequency ofthe radio-frequency signals. This frequency is, for example, of theorder of 26 MHz. Thus, the interfering image frequency is included inthe signal pass-band and thus does not need to be eliminated by such aselective filter.

The current consumption is greatly reduced, since only a doublefrequency conversion is carried out with a second not very selective lowpower-consuming filter. Moreover, the majority of the componentsoperates at a lower frequency than the frequency conversion device ofthe prior art.

The object, in addition to others, is also achieved by the RF/IFintegrated circuit for a frequency conversion device characterised inthat it includes the first mixer unit, the second pass-band filter, thesecond mixer unit, the shaping means and the majority of the oscillatingsignal generating means.

For the purposes of reducing the power consumption and size of thedevice's electronic components, the RF/IF integrated circuit can be madein CMOS technology of 0.25 μm or less. However, it could also beenvisaged to make it in biCMOS or bipolar technology, or even with CMOStechnology greater than 0.25 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects, advantages and features of the radio-frequency signalfrequency conversion device for an RF receiver will appear more clearlyin the following description of at least one embodiment illustrated bythe drawings, in which:

FIG. 1 described hereinbefore shows a radio-frequency signal frequencyconversion device according to the prior art;

FIG. 2 shows schematically the various parts forming a radio-frequencysignal receiver;

FIG. 3 shows a radio-frequency signal frequency conversion deviceaccording to the invention;

FIGS. 4 a and 4 b show graphs of the gain of the signals filtered by asecond external passive filter as a function of the frequency of thefiltered signals; and

FIG. 5 shows a graph of the gain of the second integrated filteraccording to the invention as a function of the filtered signalfrequency.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

In the following description, those elements of the frequency conversiondevice for a low power RF receiver, which are well known to thoseskilled in the art in this technical field, are only related in asimplified manner. Moreover, reference will only be made to a device fora low power GPS type receiver, even if the device can of course be usedin any other type of RF receiver.

Referring to FIG. 2, a radio-frequency signal receiver 1 isschematically shown. Said receiver 1 includes an antenna 2 for pickingup RF signals originating in particular from visible satellites tosearch and to track. In the case of a GPS receiver, it is necessary thatat least four visible satellites are tracked so that the receiver canextract the GPS data for the calculation of the position or the speed.

The RF receiver includes a frequency conversion device 3, called theRF/IF device in FIG. 2, directly connected to antenna 2 to receive theradio-frequency signals. Frequency conversion device 3 has the task oflowering the carrier frequency of the received radio-frequency signalsvia a double frequency conversion in the RF and IF circuits 4′ and 4″.The frequency conversion in circuits 4′ and 4″ is achieved as a resultof oscillating signal control means 5. These means 5 include, inparticular, a reference oscillator unit and a frequency synthesiser. Theelements forming said device forming the subject of the invention willbe explained in more detail hereinafter with reference to FIG. 3.

Device 3 provides intermediate complex IF signals which are sampled andquantified in a correlation stage 6, which is formed of severalcorrelation channels 6′. Channels 6′ operated in the correlation stagewill extract the GPS data owing to the correlation steps. Each channel6′ will generate a replica of the specific pseudo-random code of thesatellite to be tracked to correlate with the intermediate complex IFsignals. Moreover, each channel also generates a replica of the carrierfrequency to be correlated with the intermediate IF signals. Thus, assoon as each channel is locked onto the tracked satellite, it cantransmit the GPS messages via a data bus 7 to microprocessor means 8.These microprocessor means 8 will thus receive the GPS data from atleast four operating channels to be able to calculate position, speedand time-related data.

It should be noted that the intermediate IF signals are preferably, incomplex form, formed of an in-phase signal component I and aquarter-phase signal component Q at a frequency of the order of 400 kHz.The intermediate complex IF signals are represented in FIG. 2 by a lineintersected by an oblique bar defining 2 bits. It is also possible forthe in-phase signals and the quarter-phase signals to each be defined by2 bits.

The low power GPS receiver can be fitted to a portable object, such as awristwatch in order to provide, as required, position, speed and localtime data to the person wearing the watch. Since the watch has anaccumulator or battery of small size, the power consumed has to be aslow as possible during operation of the GPS receiver.

Of course, the GPS receiver could be fitted to other low consumingportable objects of small size, such as portable telephones, which arealso fitted with an energy accumulator or a battery.

With reference to FIG. 3, a preferred embodiment of frequency conversiondevice 3, forming the subject of the invention, will now be described,which is connected to an antenna 2 for receiving radio-frequencysignals. For civil applications, the carrier frequency of GPSradio-frequency signals has a value of 1.57542 GHz.

Device 3 includes first of all a first low noise type amplifier or LNA11, followed by a selective pass-band Surface Acoustic Wave (SAW) filter12. The output of filter 12 is connected to an RF/IF integrated circuit10 intended to carry out a double frequency conversion of the filteredradio-frequency signals to generate sampled and quantified intermediatesignals IF(I) and IF(Q).

As can be seen, the number of external components to the RF/IFintegrated is reduced to a minimum in order to allow enough place forassembly in a wristwatch or cellular telephone. Moreover, integratedcircuit 10 can be made in a semiconductor material, such as silicon, byCMOS technology of 0.25 μm for example. This allows a significant numberof electronic components to be integrated while also guaranteeing areduction in electric power consumption.

The RF/IF integrated circuit 10 of frequency conversion device 3 thusreceives filtered and amplified radio-frequency signals. A pre-amplifier13 at the input of the RF/IF circuit allows the sensitivity of theintegrated circuit to be increased to process the radio-frequencysignals. In a first frequency conversion operation, the outgoingradio-frequency signals from pre-amplifier 13 are mixed in a first mixerunit 14 with first high frequency signals. The first high frequencysignals are provided by a voltage-controlled oscillator VCO 20 formingpart of a frequency synthesiser on the basis of reference signalsprovided by an oscillator unit 26 and 28. Mixer unit 14 is only formed,in this embodiment, of a single mixer 14.

The signals thus generated by mixer 14 are signals whose frequency isequal to the result of a subtraction of the frequency of the first highfrequency signals from the carrier frequency of the filteredradio-frequency signals. By way of non-limiting numerical example, thecarrier frequency of the radio-frequency signals has a value of 1.57542GHz, whereas the first high frequency signals are fixed to have a valueof 1.5508 GHz. The frequency of the signals produced by first mixer 14thus have a value of 24.6 MHz which is thus approximately 64 times lessthan the radio-frequency signal carrier frequency. However, it isentirely possible to envisage having a signal frequency situated withina margin of 50 to 100 times less than that of the radio-frequencysignals.

For this first frequency conversion operation by first mixer 14, thefirst pass-band filter 12 has to be a selective passive filter, inparticular of the SAW type. Said filter 12 has to be sufficientlyselective to eliminate the image frequency of the radio-frequencysignals at the input of first mixer 14.

With a frequency of the first high frequency signals equal, for example,to 1.5508 GHz, the frequency of the signals generated by the first mixerhas a value of approximately 24.6 MHz. Thus the first selective filterhas to be capable of eliminating the image frequency having a value of1.5262 GHz (1.5508 GHz-0.0246 GHz) of the received radio-frequencysignals.

The signals produced by first mixer 14 are filtered and amplified by asecond active pass-band filter 15. This second pass-band filter does notneed to be selective like first filter 12, since the interfering imagefrequency of the signals is included within the pass-band of the GPSsignals (24.6 MHZ±1 MHz). This filter 15 can thus advantageously beintegrated in the RF/IF integrated circuit. The only restriction of thisfilter is that it has to be capable of removing the harmonic frequencyof 24.6 MHz, which is thus close to 49 MHz.

The second active pass-band filter may be well known filter by the nameof IFA-gmC. This filter has the advantage of amplifying the signalsgenerated by the first mixer around the frequency of 24.6 MHz, which isnot the case of the passive selective filters. By way of illustration,reference can be made to FIGS. 4 and 5, which show graphs of the gain asa function of the signal frequency for each type of filter.

FIGS. 4 a and 4 b show the gain curve of the signals filtered in a SAWtype selective filter around the frequency of 179 MHz cited by way ofexample in FIG. 1. In this case, the filter had to remove the imagefrequency 169.7 MHz at the input of a mixer having to generate signalsat a frequency of 4.8 MHz. It will be noted that there is a loss of atleast 4 dB on the filtered signals around 179 MHz, which is a drawback.

It would also have been possible to show the gain curve of the firstpass-band filter of the device according to the invention. Thus, thisfirst selective filter has to be configured to remove the imagefrequency of 1.5262 MHz. The complexity of such a selective filter isclear from the shape of the gain curve. This filter is thus expensiveand bulky, which is why the frequency conversion device only includesone.

FIG. 5 shows the gain curve of the signals filtered by the second activepass-band filter integrated in the RF/IF integrated circuit around thefrequency of 24.6 MHz. As can be seen in FIG. 5, the filter is notselective since the attenuation slope around the frequency of 24.6 MHzis not steep. This filter must thus be capable of removing the signalshaving the harmonic frequency of 49 MHz.

With reference again to FIG. 3, second filter 15 is followed by a secondmixer unit 16 for the second frequency conversion. This mixer unit 16 isformed of two mixers 16 a and 16 b. In mixer 16 a, second high frequencyin-phase signals I are mixed with the signals filtered by the secondfilter 15. In mixer 16 b, second high frequency quarter-phase signals Qare mixed with the signals filtered by second filter 15.

The second high frequency in-phase and quarter-phase signals areprovided by a divider register element 23 of the frequency synthesiser.This register 23 receives signals from a first divider 21 connected tovoltage-controlled oscillator 20. By way of non-limiting numericalexample, first divider 21 carries out a frequency division by 32,whereas divider register 23 carries out a division by 2. If thefrequency of the first high frequency signals has a value of 1,5508 GHz,this gives a frequency of 24.2 MHz of the second high frequency in-phaseand quarter-phase signals.

The signals generated by mixers 16 a and 16 b are signals whosefrequency is equal to the result of a subtraction of the frequency ofthe second high frequency signals from the frequency of the signalsgenerated by the first mixer. The signals generated by each of mixers 16a and 16 b have a frequency of close to 400 kHz.

Signal shaping means are placed at the output of mixers 16 a and 16 b.These means include after each mixer a low-pass filter (1.5 MHz) 17 aand 17 b, a controlled gain amplifier 18 a and 18 b and finally ananalogue/digital converter 19 a and 19 b. Thus sampled and quantifiedintermediate complex signals IF(I) and IF(Q) are generated by frequencyconversion device 3.

Each amplifier 18 a and 18 b can be controlled, in order to adjust thesignal amplification, by a level detector integrated in the RF/IFcircuit or by a correlator of the correlation stage, which receives theintermediate complex signals.

Each converter 19 a and 19 b is clocked by clock signals CLK provided bythe oscillator unit. The reference oscillator unit thus includes anoscillator 28 with a quartz crystal 29 providing reference signalsthrough an oscillator divider 26. The frequency of the signals providedby the oscillator is fixed, for example, at 16.154 MHz. The oscillatordivider 26 divides the signals by 2 for example, which gives thereference signals at the frequency of 8.08 MHz. In order to obtain theclock signals at 4.04 MHz, the frequency of the reference signals isdivided by 2 in a divider 27. These clock signals CLK are also providedto the correlation stage connected to the frequency conversion device,as well as to the microprocessor means. However, said microprocessormeans can also be clocked by the reference signals of the referenceoscillator unit.

The frequency conversion device includes a phase lock loop frequency,synthesiser partially described hereinbefore, for generating the firstand second high frequency signals. This synthesiser includes voltagecontrolled oscillator 20, two frequency dividers 21 and 22, a frequencyand phase detector 24 and a low-pass filter 25. The phase and frequencydetector compares the frequency of the signals originating fromoscillator 20, which are divided by dividers 21 and 22, with thefrequency of the reference signals generated by the reference oscillatorunit. The control signals leaving detector 24 are filtered by a low-passfilter 25 external to the RF/IF integrated circuit in order to generatea control voltage at oscillator 20 as a function of the comparison ofthe signals provided to said detector 24.

The power consumed with the configuration of the device describedhereinbefore is much lower than 30 mW, close to 20 mW. The integrationof the second filter in the RF/IF integrated circuit, as well as thedouble frequency conversion guarantees that an RF receiver including thedevice can be easily mounted in a portable object with a battery oraccumulator of small size.

From the description that has just been given of multiple variants ofthe frequency conversion device for a low power receiver, in particularof the GPS type, can be conceived without departing from the scope ofthe invention defined by the claims.

1. A radio-frequency signal frequency conversion device for generatingintermediate signals intended to be processed in a correlation stage ofa low power RF receiver of GPS type wherein the device is adapted to bemounted in a portable small sized object, the device including: a firstpass-band filter for filtering radio-frequency signals picked up by anantenna of the receiver, said first filter is a passive selective filterof the SAW type that is sufficiently selective to eliminate imagefrequencies during a first frequency conversion from receivedradio-frequency signals; oscillating signal generating means forgenerating first and second high frequency signals, the frequency of thefirst signals is higher than the frequency of the second signals,wherein the oscillating signal generating means includes a referenceoscillator comprising a quartz crystal; a first mixer unit for mixingfiltered radio-frequency signals with the first high frequency signalsin order to generate signals whose frequency is equal to the result of asubtraction of the frequency of the first signals from a carrierfrequency of the radio-frequency signals; a second pass-band filter forfiltering signals originating from the first mixer unit; a second mixerunit for mixing signals filtered by the second filter with the secondhigh frequency signals in order to generate signals whose frequency isequal to the result of a subtraction of the frequency of the secondsignals from the frequency of the signals originating from the firstmixer unit; and shaping means for signals provided by the second mixerunit for generating the intermediate signals; wherein the second filteris a not very selective active filter operable to remove a harmonicfrequency because an attenuation slope of the second filter around afirst frequency is not steep so that the active filter is capable ofremoving the harmonic frequency, and wherein the second filter isintegrated in an RF/IF integrated circuit with the first mixer unit, thesecond mixer unit, the signal shaping means, and the oscillating signalgenerating means with the exception of a control signal filteringlow-pass filter for a voltage controlled oscillator of a frequencysynthesizer and the quartz crystal of the reference oscillator unit ofthe oscillating signal generating means.
 2. The device according toclaim 1, wherein the portable small sized object is a wristwatch andwherein the second active filter filters and amplifies signals generatedby the first mixer unit.
 3. The device according to claim 1, wherein thefrequency of the signals generated by the first mixer unit is within amargin of 50 to 100 times less than a carrier frequency of theradio-frequency signals.
 4. The device according to claim 1, wherein thefrequency of the first high frequency signals is n times higher than thefrequency of the second high frequency signals, n being an integernumber selected from the range of 50 to
 100. 5. The device according toclaim 1, wherein the oscillating signal generating means includes thereference oscillator unit and a frequency synthesiser connected to thereference oscillator unit, said synthesiser providing the first andsecond high frequency signals on the basis of reference signals providedby the reference oscillator unit.
 6. The device according to claim 5,wherein all the frequency synthesiser elements, including a phase lockloop, are integrated in the RF/IF integrated circuit with the exceptionof the control signal filtering low-pass filter for a voltage controlledoscillator, wherein the oscillator is operable to generate the firsthigh frequency signals.
 7. The device according to claim 5, wherein theoscillator unit provides reference signals whose frequency is determinedby a quartz crystal and is situated between 10 and 20 MHz.
 8. The deviceaccording to claim 1, wherein the oscillating signal generating meansincludes the reference oscillator unit and a frequency synthesiserconnected to the reference oscillator unit, said synthesiser providingthe first high frequency signals on the basis of reference signalsprovided by the reference oscillator unit and the reference oscillatorunit providing the second high frequency signals.
 9. The deviceaccording to claim 1, wherein the second high frequency signals areformed of second in-phase signals and second quarter-phase signals, andwherein the second mixer unit includes a first mixer for mixing thesignals originating from the first mixer unit with the second in-phasesignals and a second mixer for mixing the signals originating from thefirst mixer unit with the second quarter-phase signals, the shapingmeans receiving the signals from the first and second mixers to provideintermediate complex signals formed of in-phase signals andquarter-phase signals.
 10. The device according to claim 9, wherein theshaping means includes, after each mixer of the second mixer unit, alow-pass filter followed by a controlled gain amplifier and ananalogue/digital converter clocked by clock signals provided by theoscillator unit, wherein one of the converters provides the in-phasesignals of sampled and quantified intermediate signals, and the otherconverter provides the quarter-phase signals of sampled and quantifiedintermediate signals.
 11. An RF/IF integrated circuit for a deviceaccording to claim 1, wherein the RF/IF integrated circuit includes thefirst mixer unit; the second active pass-band filter that is not veryselective and operates to remove the harmonic frequency different fromthe first frequency whereas the first passive filter of the SAW typeoperates to eliminate image frequencies during a first frequencyconversion from received radio-frequency signals; the second mixer unit;the shaping means; and the oscillating signal generating means with theexception of a control signal filtering low-pass filter for a voltagecontrolled oscillator of a frequency synthesiser and the quartz crystalof the reference oscillator unit.
 12. The RF/IF integrated circuitaccording to claim 11, wherein the RF/IF integrated circuit is made in asemiconductor material, and in a CMOS technology of 0.25 μm or less. 13.The RF/IF integrated circuit according to claim 11, wherein thesemiconductor material is silicon.
 14. The device according to claim 1,wherein the frequency of the signals generated by the first mixer unitis 64 times less than a carrier frequency of the radio-frequencysignals.
 15. The device according to claim 1, wherein the frequency ofthe first high frequency signals is n times higher than the frequency ofthe second high frequency signals, n being an integer number equal to64.
 16. The device according to claim 1, wherein the second activefilter filters and amplifies signals generated by the first mixer unit.